A light emitting apparatus has recently been actively researched in which a semiconductor light emitting element such as a light emitting diode (LED) or a semiconductor laser diode (LD) is used as an excitation light source and fluorescence generated by irradiating a light emitting section containing a fluorescent material with excitation light generated from such an excitation light source is used as illumination light.
Such a light emitting apparatus is exemplified by a light source unit disclosed in Patent Literature 1. The light source unit irradiates a fluorescent material with light emitted from a light source section and the irradiation causes the fluorescent material to generate diffused light. A reflecting mirror, which is provided between the light source section and the fluorescent material, carries out light distribution control with respect to the diffused light from the fluorescent material so as to cause the diffused light to be substantially parallel and to be emitted forward.
As described earlier, according to the light source unit, the reflecting mirror is provided between the light source section and the fluorescent material. Therefore, the reflecting mirror has a light transmitting section which is a hole via which light is transmitted from the light source section toward the fluorescent material. The light emitted from the light source section enters the reflecting mirror from the light source section side, is transmitted through the light transmitting section, and goes toward the fluorescent material.
The light transmitting section which transmits the light emitted from the light source section is merely a hole which is through the reflecting mirror. Therefore, of the diffused light generated by the fluorescent material, light going to the light transmitting section is transmitted through the light transmitting section which is the opening. Namely, a part of the diffused light enters the reflecting mirror from the fluorescent material side, is transmitted through the light transmitting section, and goes toward the light source section. Namely, the part of the diffused light leaks out of the light transmitting section. Therefore, the light source unit raises a problem of causing a decrease in efficiency with which the diffused light generated by the fluorescent material is used.
In view of such a problem, it can be said that it is preferable to provide the light transmitting section with a wavelength-selective reflecting mirror or band-pass filter which transmits the light emitted from the light source section but does not transmit the diffused light generated by the fluorescent material, i.e., reflects the diffused light.
For example, according to a light emitting apparatus disclosed in Patent Literature 2, an emission end face of a fiber optical waveguide is provided with a reflecting mirror which has a high reflectance with respect to a wavelength of semiconductor laser light and has a low reflectance with respect to a wavelength of emission by the fluorescent material. Such a reflecting mirror transmits light generated by the fluorescent material but reflects the semiconductor laser light on the emission end face of the fiber optical waveguide.
According to a semiconductor light emitting apparatus disclosed in Patent Literature 3, a cylindrical cap surrounds a semiconductor light emitting element, and a wavelength conversion substance (a fluorescent material) is provided outside the cap. Light emitted from the semiconductor light emitting element goes through a through-hole opened in a main part of the cap, and the wavelength conversion substance provided outside the cap is irradiated with the light. The through-hole is provided with a light selecting filter. As such a light selecting filter, a wavelength-selective band-pass filter is used which transmits the light emitted from the semiconductor light emitting element but does not transmit light having been subjected to wavelength conversion by use of the wavelength conversion substance.
In order to prevent a leak of the diffused light from the light transmitting section in the light source unit of Patent Literature 1, it can be expected to be effective to provide the light transmitting section with the reflecting mirror of Patent Literature 2 or the band-pass filter of Patent Literature 3. It is only necessary that wavelength selectivity of the reflecting mirror or the band-pass filter be controlled to transmit the light emitted from the light source and reflect the diffused light from the light emitting section.
Normally, it is common to use, as the reflecting mirror of Patent Literature 2 or the band-pass filter of Patent Literature 3, not a single layer film but a multilayer film which is constituted by a plurality of layers of films. According to such a multilayer film, in a case where kinds of films of respective layers and optical path lengths in the respective layers are combined most suitably, desired wavelength selectivity can be obtained.
Note that an optical path length is obtained by multiplying a distance which light has actually traveled in each layer (hereinafter may be referred to as a “propagation distance”) by a refractive index of a substance constituting a film in the each layer. Namely, the optical path length is defined by the following equation:Optical path length=propagation distance×refractive index